The present invention is directed to low-noise amplifiers for use in global radio frequency receivers.
With the advent of third generation wireless systems using wide band CDMA (W-CDMA) technology, mobile receivers will typically be operating in a continuous fashion. This is to be distinguished from prior generation wireless receivers such as GSM receivers which are turned on only for a fraction of the time. As a result, there is a need for a new type of differential low-noise amplifier (LNA) arrangement for such mobile radio frequency receivers and in particular, to maintain power consumption of such receivers at low levels in order to increase operating time of the mobile appliance with limited battery capacity.
Thus prior art, conventional RF receivers use LNA designs that consume power at essentially a constant rate regardless of the input signal. In the prior art, conventional radio frequency (RF) receivers use LNAs with a constant current source to provide power to the LNA.
Linear amplifiers with signal-dependent power consumption are widely used for certain types of audio amplifiers and in particular, Class AB amplifiers, as well as for other low-frequency applications. Although there are some commercially available LNAs which allow the current to change through use of a control signal in order to trade-off power consumption against linearity of the LNA, there is no known solution which adapts power consumption to the signal level automatically. In European application 0 999 649, a method and arrangement for linearizing a radio receiver is disclosed. In this prior art document, the mobile receiver monitors signal strength on the received channel as well as neighboring channels so as to determine the quality of the detected signal through one of several means. If the signal strength is satisfactory, the supply currents at the front end of the amplifier, and at a first mixer of the receiver are kept relatively small. If however, the signal strength falls below a predetermined value or if neighboring channels exceed a predetermined value, the supply currents are increased so as to obtain better linearity of the amplifier. This European application does not however disclose a solution to minimize power consumption of a high-frequency, low-noise amplifier; and in particular to minimize power consumption when the input signal is below a predetermined value and to increase the current flow of the low-noise amplifier and increase power consumption when input power levels are increased, thereby maintaining better linearity of the LNA when high input signals are present.
The described signal-dependent current controlled low-noise amplifier consumes only approximately twenty-five percent of its maximum current in the absence of a high input signal, which contains the desired signal and possibly interfering signals. In order to achieve this result, a differential low-noise amplifier is described having a current source which is controlled by the magnitude of the input signal. The magnitude of the input signal is obtained through an active rectifier which uses emitter followers for purposes of obtaining such active rectification.
In the absence of a high input signal, the control voltage of the current source is essentially constant and thus the differential amplifier operates at its quiescent current.
At higher input signal levels as a result of a higher value of the desired signal or higher values of interfering signals, the control voltage for the current source follows the instantaneous magnitude of the RF input signal, thereby increasing the output current of the current source. The current-control mechanism thereby generates a variation in the total current at twice the input signal frequency which would be twice the input signal frequency of either the desired signal or the desired signal in combination with interfering signals. The two-times the input frequency is due to the active rectification of the input signal. The current control outputs representing variations in the output current of the current source appear as outputs to the low-noise amplifier in a common mode when the amplifier is a differential low-noise amplifier. The amplified desired signal is seen at the output of the differential LNA in differential mode thereby; easily differentiating that signal from the common mode variation of the output signal.
The gain of the differential amplifier typically decreases for higher input signal levels due to the shape of the voltage/current transfer curve for such differential amplifiers; but the gain of such differential amplifiers is proportional to the total current driving the LNA. Thus increasing the total current by increasing the output current of the current source effectively overcomes the decrease in the gain of the differential amplifier otherwise associated with increased input signal levels.
Thus the optimization goal for the low-noise amplifier circuit according to the present invention is to compensate for the gain loss of the differential amplifier through use of additional current from the current source. At higher signal levels the differential amplifier operates as a limiting amplifier and thus the gain is produced by the current source alone.
In particular since the current gain of the transistors forming the differential amplifier is approximately constant, the instantaneous base current of these transistors is proportional to the instantaneous current through the current source. At high input signal amplitudes (and therefore high currents), a multiple of the quiescent base current flows through either one of the two differential amplifier transistors, depending on the polarity of the RF input signal. The other transistor is then completely blocked. Therefore a special bias network is used which provides a negative return path for the increased signal current flowing through the input signal source. This bias network is able to deliver varying base currents to the differential amplifier while keeping the additional voltage drop at high peak base currents at a sufficiently low value. In addition, the small signal impedance associated with the bias network is high and therefore only a small ratio of the input signal energy is lost leaking into the bias network.